The energy and power density of electrochemical rechargeable batteries have increased significantly in recent years. Meanwhile, safe operation of these high energy devices has become a stringent requirement especially for applications in large scale renewable energy storage, electric drive vehicles, and next generation portable electronics. For the widely used lithium-ion rechargeable batteries in particular, safety issues are often associated with the formation of dendritic lithium on the negative electrode. In the lithium metal rechargeable battery, lithium dendrites gradually grow on the surface of the lithium metal electrode during discharge-recharge cycles. These dendrites can penetrate through the polymer separator to form a short circuit between the positive and negative electrodes, which can initiate exothermic reactions between the electrodes and the flammable organic electrolyte. The excessive heat released in these reactions often induces fire and even explosions. Due to the inherent instability of lithium metal deposition, research shifted to lithium-ion based batteries during the 1980s. Current commercial lithium-ion batteries utilizing graphite as a negative electrode exhibit an approximately ten-fold lower specific charge capacity but safer operation compared to lithium metal electrodes; however, despite this development, lithium dendrite formation has not been entirely inhibited in graphite anodes either, especially when cycled at high current densities, under overcharge conditions, or at low temperatures.
In the past few decades, multiple approaches have been employed to address this safety problem, with suppression of lithium dendrites as a common strategy. Typical examples are the modification of the electrolyte solvent and solute, re-engineering of the surface morphologies and coating processes of the electrodes, and theoretical models of the mechanisms leading to dendrite formation. Unfortunately, despite intense efforts for several decades, it seems difficult to completely eliminate dendrite formation by current approaches since the lithium re-deposition process is non-uniform and predisposed to formation of dangerous lithium dendrites.
It is against this background that a need arose to develop the separators and electrodes described herein.